Air pollution in the urban and industrial areas is a severe problem of the nation, due to the exhaust products of the factories and automobiles. It is further aggravated with the use of low quality fuels, caused by the perennial energy shortage. While the fundamental solutions would involve formidable expenditures and longterm technological development, there is an urgent need for on-site, low cost systems that can purify the air for use in enclosed living spaces (automobiles, homes, offices, hospitals etc.) The current state-of-the-art method for air purification is effected by using filters containing substances such as activated charcoal and a variety of chemicals. The shortcomings of such devices are twofold: First, they can only remove a limited number of pollutants, and secondly, the process is irreversible, therefore periodic replacement of the devices is necessary which contributes to the high cost of this type of system. Life supporting systems and artificial enviroments based upon pressurized pure air/oxygen bottle supply have been used in manned space exploration, diving, and medical applications. The main problem in extending this technology for general use is again the factor of high cost. It is estimated that more than 90% of the cost is due to the transportation and distribution of the heavy bottles used to contain the high pressure gaseous material from the factory to the site of application.
This invention proposes to produce pure air or oxygen at the site of application by cryogenic techniques, thereby achieving cost effectiveness. The cryogenic technique is chosen, because it is much more efficient than other techniques such as, for example electrolysis of water. The technical background of the invention is easy to appreciate as briefly summarized below:
The first important advantage of the system is the efficiency of cryogenic refrigeration, which includes a compressor, defined as the cooling power divided by the power required to operate the compressor. It depends somewhat upon the size of the system. For instance, a large scale refrigerator such as is used in an industrial air liquefier has an efficiency of the order of 30%. Highly portable, light weight refrigerators for aerospace application have efficiencies on the order of 2 to 4 percent. Thus, the efficiency of a system for automobiles, homes, etc. will be somewhere in between these values. This relative inefficiency is offset by the fact that the human respiratory need for air/oxygen is comparatively low. It is estimated that a normal person consumes about 0.063 pounds of oxygen per hour. Based upon the ideal work of obtaining oxygen gas through liquefaction of oxygen at 170 BTU/lb and the current cost of energy, the operating cost per person per hour of the system is less than one cent. Thus it is economical.
The second important advantage of the cryogenic filter is in the thermal properties of air (oxygen/nitrogen) and the major gaseous pollutants contained in it. The melting/boiling points in centigrades (.degree.C.) of these substances are listed as follows:
TABLE I ______________________________________ N.sub.2 O.sub.2 NO NO.sub.2 N.sub.2 O CO ______________________________________ Melting point, .degree.C. -209.8 -218.4 -163.6 -9.3 -102.4 -207 Boiling point, .degree.C. -195.8 -182.9 -151.8 21.3 -88.5 -190 ______________________________________ NH.sub.3 SO.sub.2 SO.sub.3 H.sub.2 S H.sub.2 S.sub.2 CO.sub.2 ______________________________________ Melting point, .degree.C. -74 -75.5 16.8 -62.0 -82.9 -78.5 Boiling point, .degree.C. -30.9 -10.0 44.8 -42.0 -61.8 -- ______________________________________ HF HCl CF.sub.4 CH.sub.4 C.sub.2 H.sub.4 ______________________________________ Melting point, .degree.C. -92.3 -112 -184 -184 -169.4 Boiling point, .degree.C. 19.4 -83.7 -128 -161.5 -103.9 ______________________________________
From these data, it is evident that if the polluted air containing these gaseous pollutants is being filtered by a cryogenic filter at a cryogenic temperature between -180.degree. and -184.degree. C., all the pollutants perhaps besides carbon monoxide (CO) will be condensed on the filter in liquid or in solid form, and thus removed from the air. There are also evidences (reference I) that due to its larger electrical dipole moment, the CO molecule shows a much larger physical adsorption than nitrogen and oxygen over this temperature range in a number of porous materials such as activated carbon, therefore it can be also effectively removed. It is also well-known that hydrocarbon emissions from automobiles, such as parafins, olefins and aromatics, have larger molecular weights, and consequently higher boiling and melting points. They can be handily condensed and removed from the air by the cryogenic filter.